An overview of terms and topics that require explanation.

A   B   C   D   E   F   G   H   I   J   K   L   M   N   O   P   Q   R   S   T   U   V   W   X   Y   Z


Bioreactor / SBR tank

In a fully biological small wastewater treatment plant, a bioreactor (SB reactor) is installed downstream of the mechanical primary treatment. The actual biological treatment takes place in the SBR tank.
Short aeration and rest phases alternate in a controlled purification process.



The biochemical oxygen demand after 5 days indicates the amount of oxygen required within 5 days to biodegrade existing organic substances in the wastewater (by microorganisms). The specification of the value results from the wastewater ordinance.




The chemical oxygen demand indicates the amount of oxygen required to break down all the organic substances present in the wastewater - including the non-biodegradable substances. The specification of the value results from the wastewater ordinance.


Domestic wastewater

All wastewater that flows from private households to a small wastewater treatment plant.


Excess sludge

Surplus sludge is the excess activated sludge that continuously accumulates during the activated sludge process.




After the decomposition of consumables and nutrients, the hygienization of the wastewater is increasingly aimed at in order to eliminate pathogens in the biologically treated wastewater. For this purpose, the biologically treated wastewater is irradiated with UVC light, for example. In this way, the wastewater treated in small wastewater treatment plants achieves a better water quality, which in turn allows it to be discharged into sensitive areas (karst or water protection areas).



Industrial wastewater

This includes industrial wastewater from manufacturing companies, restaurants, etc.


Multi-chamber pits

Multi-chamber pits (multi-chamber septic tanks) represent the original method of wastewater treatment. A multi-chamber pit usually consists of two to three chambers, through which the solids content is effectively retained. Multi-chamber pits usually have a size of 0.3 m³/E (minimum volume 3 m³) and must be emptied at least once a year. Multi-chamber cesspools with a size of 1.5 m³/E (minimum volume 6 m³) are counted as multi-chamber septic tanks among the biological wastewater treatment plants. Multi-chamber cesspools cannot meet today's requirements and must therefore be retrofitted to become biological small wastewater treatment plants.


Monolithic tank

The tank is made of one piece. The counterpart to this in a concrete tank is, for example, the ring construction. The advantage of monolithic tanks is that they have good stability and water tightness.



Nitrogen reduction

Elimination of nitrogen contained in sewage water. Nitrogen reduction takes place biologically with the help of certain strains of microorganisms. Plants with additional nitrification use particularly intensive aeration to create optimum living conditions for nitrifying bacteria, which convert ammonium (NH4) into nitrate (NO3). In systems that are also designed for additional denitrification, short aeration pulses at the beginning of the aeration phase circulate the water and stimulate the denitrifying bacteria, which convert nitrate (NO3) into elemental nitrogen (N2).



Population values (PE) calculation for residential buildings


Small wastewater treatment plants for residential buildings are to be dimensioned according to the number of inhabitants.

  • At least four PE per residential unit >60 m2
  • At least two PE per residential unit <60 m2
  • If more than three residential units or buildings are combined (group solutions), these minimum requirements can be deviated from for these additional residential units.


Permit under water law

A permit issued by city and state government agencies (e.g., Lower Water Authority) authorizing the discharge of treated wastewater into surface waters or groundwater.


Primary sludge

Solid components of wastewater that settle in the first chamber of a small wastewater treatment plant.


Preliminary clarification

The aim of primary sedimentation is to separate coarse material from the wastewater by gravity. This means that the wastewater is pre-cleaned. Preliminary clarification relieves the downstream biological stage.




Rainwater must not be fed into the small wastewater treatment plant. Dilution of the wastewater has a negative effect on the cleaning effect. In addition, it can hydraulically overload the plant.


Ring construction

Concrete tanks can be erected on site using the ring construction method. Here, ring for ring is glued on top of each other until the desired tank size is reached. The advantage is that transport is easier. The disadvantage is that the tightness between the individual rings can decrease.



Small wastewater treatment plants


Wherever a connection to the sewer system is not economically feasible, small wastewater treatment plants are installed. Small wastewater treatment plants are systems for the treatment of domestic wastewater collected by the separation process with a daily wastewater volume of up to 8 m3.

Not allowed to be fed:

  • Commercial and agricultural wastewater (insofar as it is not comparable with domestic wastewater)
  • Condensates from fireplaces with pH-values below 6.5, substances that interfere with the operation of the sewage treatment plant
  • Effluent water from swimming pools
  • Extraneous water and cooling water
  • Precipitation water

Sewage sludge

The solids produced by sedimentation (settling due to gravity) during mechanical and/or biological wastewater treatment.


Secondary sedimentation

Last stage of the wastewater treatment plant. In this stage, the entrained sludge settles before the treated water leaves the plant. In SBR plants, the secondary sedimentation tank is omitted. Settling takes place directly in the activated sludge tank or SBR tank.


Switch cabinet

The switch cabinet is the head of our small sewage treatment plant. It contains the control system, the compressor, the solenoid valves, but also the additional components, such as the phosphate pump or the modem for remote monitoring. A distinction is made between an outdoor cabinet (installed outdoors) and an indoor cabinet (installed in a building, house or cottage).




Underloading is a frequently discussed issue with small wastewater treatment plants. Some types of plants have difficulties to keep the effluent values when influent quantities or loads fluctuate. In practice, however, this is often the case! In 2011, we subjected the KLARO plant to a voluntary endurance test to demonstrate that our plant can cope well with various underload scenarios that go beyond the normal test program. For this purpose, the plant was run for several weeks at various low loads (50%, 25%, 0%).  The effluent values hardly deviated from those in nominal operation and were always well below the usual limits.



Wastewater treatment plants

From 50 PE to 5.000 PE one speaks of small wastewater treatment plants. Plants for population equivalents of less than 50 are small wastewater treatment plants.


Walkable / Driveable

For a cone (cover), a distinction is made between walkable (5to.) or drivable. With trafficable there is a further differentiation between 12,5to. (passenger car) and 40to. (TRUCK). Depending on the positioning of your small sewage treatment plant you have to decide between walkable or drivable.

Wastewater Ordinance

The discharge of wastewater into bodies of water is subject to various requirements that are regulated in the Wastewater Ordinance. In the area of small wastewater treatment plants, the AbwV regulates the limit values, which are defined in more detail within the discharge classes.

Success can be measured

KLARO in numbers
- You can count on us.


Users worldwide


In over 80 countries


Specialists in Bayreuth